2011MNRAS.414.1875H

Simple pure luminosity evolution (PLE) models, in which galaxies brighten at high redshift due to increased star formation rates (SFRs), are known to provide a good fit to the colours and number counts of galaxies throughout the optical and near-infrared. We show that optically defined PLE models, where dust reradiates absorbed optical light into infrared spectra composed of local galaxy templates, fit galaxy counts and colours out to 8 µm and to at least z≈ 2.5. At 24–70 µm, the model is able to reproduce the observed source counts with reasonable success if 16 per cent of spiral galaxies show an excess in mid-IR flux due to a warmer dust component and a higher SFR, in line with observations of local starburst galaxies. There remains an underprediction of the number of faint-flux, high-z sources at 24 µm, so we explore how the evolution may be altered to correct this. At 160 µm and longer wavelengths, the model fails, with our model of normal galaxies accounting for only a few percent of sources in these bands. However, we show that a PLE model of obscured AGN, which we have previously shown to give a good fit to observations at 850 µm, also provides a reasonable fit to the Herschel/BLAST number counts and redshift distributions at 250–500 µm. In the context of a ΛCDM cosmology, an AGN contribution at 250–870 µm would remove the need to invoke a top-heavy IMF for high-redshift starburst galaxies.